One of the key problems in constructing and maintaining cellular radio networks is the limited scope of available radio spectrum. Careful planning of the use of radio frequencies aims at utilizing the available frequencies as efficiently as possible but simultaneously at minimizing co-channel interference and adjacent channel interference. By means of various models, available frequencies are divided into different cells so as to minimize the interference occurring in radio connections and thus maximize the network capacity. In a cell repeat pattern, the same or adjacent frequencies must not be too close to one another, because this causes excessive interference in the system. On the other hand, the tighter the repeat pattern, the more efficient the usage of the frequencies.
In known mobile systems, the allocation of a radio channel to a given subscriber terminal generally depends on whether or not a radio cell has an available traffic channel at that instant. In the simplest case, if said radio cell has a free radio channel, it is allocated to the subscriber terminal when necessary. More sophisticated channel allocation methods have also been developed. The quality of an idle radio channel may be taken into account. For example in a GSM system (Global System for Mobile Telecommunications), when allocating an uplink radio channel, the interference power level of idle radio channels is measured and a channel with the lowest interference level is allocated to a terminal. Since the GSM system is time-divisional, a traffic channel which comprises a time slot on a given frequency channel is allocated to the subscriber terminal.
U.S. Pat. No. 6,799,044 discloses a dynamic channel allocation method where a carrier-to-interference ratio of available channels is evaluated. In the solution, a channel and a frequency are dynamically selected on the basis of the evaluated ratio. However, the proposed solution does not take possible interference rejection into account and it cannot be used in systems employing fixed frequency planning.